When dispensing viscous fluids such as certain lubricants, adhesives sealants and the like, it is often necessary to apply the material to the surface of a workpiece in a bead containing a desired amount of material per unit length. In high production processes or where the bead of material must be positioned with accuracy, robot arms are often used to apply the material by rapidly guiding a dispensing nozzle in a programmed pattern over the surface of the workpiece. Depending on the application, the fluid being dispensed may either be projected some distance from the nozzle in a high velocity stream or extruded from the nozzle at lower velocity with the nozzle located closer to the workpiece. In either case, the amount of material applied per unit of lineal distance along the bead will vary according to both the flow rate of material discharged from the dispensing nozzle and the speed of the nozzle with respect to the workpiece.
For example, in the automotive industry it is necessary to apply a uniform bead of sealant around the periphery of the inside surface of automobile doors before joining the inside panel to the door. Along long, straight portions of the pattern, a robot arm can move the nozzle quickly. However, where the desired bead pattern changes direction abruptly, such as around the corners of a door panel, the robot arm must be slowed down to achieve a required bead positioning accuracy. It can be appreciated that if the flow rate of the dispensed fluid material is held fixed, the amount of material in the applied bead will increase as the robot arm is decelerated to negotiate changes in direction and will decrease as the robot arm is accelerated.
In the prior art, one attempt to deal with this problem has been to apply a toolspeed signal emanating from the robot controller to a voltage-controlled D.C. motor drive to control the speed of a ball screw mechanism driving the plunger of a shot pump filled with fluid. The shot pump is connected to the dispensing nozzle on the robot arm by way of a length of flexible hose. The toolspeed signal applied to the D.C. drive varies with the speed of the nozzle relative to the workpiece. As the rate of travel of the shot pump plunger changes, so too does the flow rate from the nozzle. Thus, the rate at which fluid is dispensed is controlled in open-loop fashion according to the speed of the nozzle.
Such a system suffers a number of deficiencies. First, it is inherently slow to respond. Therefore, only limited control of bead size is possible. In addition to the delays associated with the response of the D.C. drive and mechanical system driving the plunger, the flexible hose connected between the shot pump and the nozzle carried by the robot arm introduces significant response lag into the system. With a hose only 10 feet long, and depending on supply pressure and the characteristics of the fluid being dispensed, it may take a second or more for a change in pressure at the shot pump to be reflected in a corresponding change in flow at the nozzle. Thus, very precise control of bead size is difficult particularly during rapid changes in the speed of the robot arm. In addition to these performance limitations, such systems have other practical disadvantages. The shot pump itself should be capable of holding at least as much material as required to be applied to an entire workpiece. Accordingly, the pump and its associated mechanical drive are too bulky and massive to be mounted on the robot arm with the dispensing nozzle. The mechanical components and D.C. drive controls together may weigh up to several hundred pounds. Further, such a system is expensive to maintain and occupies a significant amount of production floor space.
Another type of system known in the prior art uses a more compact dispenser having a motor driven metering valve which receives a continuous supply of material by way of a flexible hose. The dispenser is mounted on the robot arm and includes a servomotor or stepper motor which controls the metering valve to adjust the flow in accordance with the speed of the dispensing nozzle as indicated by a toolspeed signal emanating from the robot. Closed-loop control of flow is effected by a feedback signal indicative of material flow deriving, at some point in the system remote from the dispensing nozzle. This feedback signal may be derived by sensing the displacement of the supply pump using an LVDT or potentiometer connected to the crosshead of the pump or by using a positive displacement flowmeter connected in line with the flexible hose which feeds the dispenser. In addition to this main control loop, such a system can incorporate a pressure sensor at the nozzle of the dispenser to shut off under specified conditions as described in European patent application No. 85-104,127.7. This reference discloses the use of one or more pressure sensors located in the wall of the dispensing nozzle to derive a pair of signals, one of which is used to indicate the presence of bubbles, the other of which indicates the flow of the liquid. The patent states that the latter signal can be derived for example from a pair of contacts connected to an elastic pressure-transmitting element which keeps the contacts closed as long as the pressure at the nozzle exceeds a certain value. In the event a clog develops in the flow channel, the flow signal can be used to initiate a shutdown of the system or provide an indication. Similar action can be taken should a bubble be sensed at the nozzle.
This type of system also has significant performance limitations. Even though the material being dispensed is metered by a dispenser mounted on the robot arm rather than from a remote metering device such as the shot pump system described above, the response time of the system is still relatively slow. As a consequence, the ability of the system to control bead size is limited, especially during rapid changes in the relative speed between the dispenser nozzle and the workpiece.